The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and, more particularly, to a multicolor image forming apparatus of the type forming a multicolor toner image on an image carrier and then transferring it to a recording medium at a time.
With a multicolor image forming apparatus, it is possible to form a multicolor toner image on an image carrier by use of developers of different colors. A nonmagnetic toner, or one component type developer, is advantageously used with developing means for the second color since it reduces the size and cost of the apparatus and is easy to color. It has been customary to cause this kind of toner to deposit on a developer carrier in a layer and face, but not contact, an image carrier, thereby effecting development. This development does not disturb a toner image of a first color existing on the image carrier.
The image forming apparatus has, for example, a plurality of developing means arranged around an image carrier. In this kind of apparatus, developing means assigned to the first color stores a chromatic toner, which constitutes a two component type developer in combination with a carrier. Developing means assigned to the second color and located downstream of the above-mentioned developing means stores a black toner, or one component type developer. The black toner is charged to a polarity opposite to the polarity of the chromatic toner. In the downstream developing means, the toner is deposited on a developer carrier in a thickness of 30 .mu.m to 500 .mu.m. In the event of development, an AC bias voltage is applied to the developer carrier of the downstream or second developing means for generating an AC electric field which effects development by the toner. While development is not effected, a bias voltage which causes the chromatic toner of the upstream or first developing means to develop an image is applied to the developer carrier of the second developing means. This kind of scheme is taught in, for example, Japanese Patent Laid-Open Publication No. 63-60471.
However, the apparatus described above has a problem that at the time of development in the second color, the toner of second color moves back and forth between the latent image surface of the image carrier and the surface of the developer carrier while hitting against the latent image surface due to the AC bias. Such a toner is apt to disturb a toner image formed on the image carrier in the first color. Moreover, the toner of first color is apt to move back and forth together with the toner of second color while hitting against the surface of the developer carrier, entering the developing means assigned to the second color. As a result, the toner of second color stored in the developing means becomes turbid.
In light of this, there has been proposed a multicolor image forming apparatus which applies a DC bias voltage for development in the second color. The DC bias causes a nonmagnetic toner to fly toward the image carrier, thereby obviating color mixture. For example, a plurality of developing means are arranged around an image carrier having a 35 .mu.m to 90 .mu.m thick photoconductive layer which has a capacitance of 20 pF/cm.sup.2 to 170 pF/cm.sup.2 and is made of selenium or arsenic selenide. Charging, exposing and developing steps are repeated a plurality of times to form a composite color image on the same image carrier. In the developing means assigned to the second color, a gap less than 250 .mu.m is formed between a developer carrier and the image carrier. A DC bias voltage is applied to the developer carrier to execute non-contact development by using a thin toner layer. At this instant, the other developer carriers not contributing to the development are held inoperative, i.e., toners are deposited thereon at the outside of their imaging regions. For this type of apparatus, a reference may be made to Japanese Patent Laid-Open Publication No. 63-63061. This document includes an embodiment using an image carrier implemented by an organic photoconductor having a 15 .mu.m to 50 .mu.m thick photoconductive layer, charging means in the form of a scorotron charger, reversal development, a potential contrast greater than 400 V, and a 5 .mu.m to 30 .mu.m thick toner layer deposited on the photoconductor.
Another type of multicolor apparatus image forming apparatus includes a plurality developing means facing, but not contacting, a recording medium. In this type of apparatus, first or upstream developing means includes a developer carrier to which a DC-biased AC bias is applied for black development. The developer carrier is rotated in the same direction as, but at a higher peripheral speed than, the recording medium. Second and successive developing means each includes a developer carrier to which only a DC bias is applied for color development. Such an apparatus is disclosed in, for example, Japanese Patent Laid-Open Publication No. 63-85578. This apparatus, however, has a drawback that when the toner is caused to fly under the electric field generated by the DC bias, cohered toner particles locally come off in low contrast portions, resulting in a granular image. Another drawback is that in latent line images, the edge electric fields of the latent images run around to the image carrier side, preventing thin lines from being reproduced.